A well-known example occurs in the opening sequence of the Star Trek television series: to boldly go where no man has gone before; the adverb boldly splits the infinitive to go. More rarely, more than one word splits the infinitive in a compound split infinitive, as in: The population is expected to more than double in the next ten years.

As the split infinitive became more common in the 19th century, some grammatical authorities sought to introduce a prescriptive rule against it. The construction is still the subject of disagreement among English speakers as to whether it is grammatically correct or good style: "No other grammatical issue has so divided English speakers since the split infinitive was declared to be a solecism in the 19c [19th century]: raise the subject of English usage in any conversation today and it is sure to be mentioned."[1] However, most modern English usage guides have dropped the objection to the split infinitive.[2]

In Old English, infinitives were single words ending in -n or -an (compare modern Dutch and German-n, -en). Gerunds were formed using "to" followed by a verbal noun in the dative case, which ended in -anne or -enne (e.g. tō cumenne = "coming, to come").[3] In Middle English, the bare infinitive and the gerund coalesced into the same form ending in -(e)n (e.g. comen "come"; to comen "to come"). The "to" infinitive was not split in Old or Early Middle English.

The first known example of a split infinitive in English, in which a pronoun rather than an adverb splits the infinitive, is in Layamon's Brut (early 13th century):

This may be a poetic inversion for the sake of meter, and therefore says little about whether Layamon would have felt the construction to be syntactically natural. However, no such reservation applies to the following prose example from John Wycliffe (14th century), who was fond of splitting infinitives:[6]

For this was gret unkyndenesse, to this manere treten there brother.[7]

However, it was especially in colloquial use that the construction experienced a veritable boom. Today, according to the American Heritage Book of English Usage, "people split infinitives all the time without giving it a thought."[10] In corpora of contemporary spoken English, some adverbs such as always and completely appear more often in the split position than the unsplit.[13]

Traditional grammarians have suggested that the construction appeared because people frequently place adverbs before finite verbs. George Curme writes: "If the adverb should immediately precede the finite verb, we feel that it should immediately precede also the infinitive…"[14] Thus if one says:

She gradually got rid of her teddy bears. and

She will gradually get rid of her teddy bears.

one may, by analogy, wish to say:

She wants to gradually get rid of her teddy bears.

This is supported by the fact that split infinitives are often used as echoes, as in the following exchange, in which the riposte parodies the slightly odd collocation in the original sentence:

Child: I accidentally forgot to feed the hamster.

Parent: Well, you'll have to try harder not to "accidentally forget", won't you?

Here is an example of an adverb being transferred into split infinitive position from a parallel position in a different construction.

This is an extension of the subject pronoun (you all). However an object pronoun, as in the Layamon example above, would be unusual in modern English, perhaps because this might cause a listener to misunderstand the to as a preposition:

*And he called to him all his wise knights to him advise.

Other parts of speech would be very unusual in this position. However, in verse, poetic inversion for the sake of meter or of bringing a rhyme word to the end of a line often results in abnormal syntax, as with Shakespeare's split infinitive (to pitied be, cited above), in fact an inverted passive construction in which the infinitive is split by a past participle. Presumably, this would not have occurred in a prose text by the same author.

It was not until the very end of the 19th century that terminology emerged to describe the construction. According to the main etymological dictionaries, the earliest use of the term split infinitive on record dates from 1897, with infinitive-splitting and infinitive-splitter following in 1926 and 1927 respectively. The now rare cleft infinitive is slightly older, attested from 1893.[16] The term compound split infinitive is not found in these dictionaries and appears to be very recent.

This terminology implies analysing the full infinitive as a two-word infinitive, which not all grammarians accept. As one who used "infinitive" to mean the single-word verb, Otto Jespersen challenged the epithet: "'To' is no more an essential part of an infinitive than the definite article is an essential part of a nominative, and no one would think of calling 'the good man' a split nominative."[17] However, no alternative terminology has been proposed.

Although it is sometimes reported that a prohibition on split infinitives goes back to Renaissance times, and frequently the 18th century scholar Robert Lowth is cited as the originator of the prescriptive rule,[18] such a rule is not to be found in Lowth's writing and is not known to appear in any other text prior to the mid-19th century.

Possibly the earliest comment against split infinitives was by an anonymous American in 1834:

I am not conscious, that any rule has been heretofore given in relation to this point [...] The practice, however, of not separating the particle from its verb, is so general and uniform among good authors, and the exceptions are so rare, that the rule which I am about to propose will, I believe, prove to be as accurate as most rules, and may be found beneficial to inexperienced writers. It is this :—The particle, TO, which comes before the verb in the infinitive mode, must not be separated from it by the intervention of an adverb or any other word or phrase; but the adverb should immediately precede the particle, or immediately follow the verb.[19]

In 1840, Richard Taylor also condemned split infinitives as a "disagreeable affectation",[20] and in 1859, Solomon Barrett, Jr., called them "a common fault".[21] However, the issue seems not to have attracted wider public attention until Henry Alford addressed it in his Plea for the Queen's English in 1864:

A correspondent states as his own usage, and defends, the insertion of an adverb between the sign of the infinitive mood and the verb. He gives as an instance, "to scientifically illustrate". But surely this is a practice entirely unknown to English speakers and writers. It seems to me, that we ever regard the to of the infinitive as inseparable from its verb. And, when we have already a choice between two forms of expression, "scientifically to illustrate" and "to illustrate scientifically," there seems no good reason for flying in the face of common usage.[22][23]

Others quickly followed, among them Bache, 1869 ("The to of the infinitive mood is inseparable from the verb");[24] William B. Hodgson, 1889; and Raub, 1897 ("The sign to must not be separated from the remaining part of the infinitive by an intervening word").[25]

Even as these authorities were condemning the split infinitive, others were endorsing it: Brown, 1851 (saying some grammarians had criticized it and it was less elegant than other adverb placements but sometimes clearer);[26] Hall, 1882; Onions, 1904; Jespersen, 1905; and Fowler and Fowler, 1906. Despite the defence by some grammarians, by the beginning of the 20th century the prohibition was firmly established in the press and popular belief. In the 1907 edition of The King's English, the Fowler brothers wrote:

"The 'split' infinitive has taken such hold upon the consciences of journalists that, instead of warning the novice against splitting his infinitives, we must warn him against the curious superstition that the splitting or not splitting makes the difference between a good and a bad writer."

In large parts of the school system, the construction was opposed with ruthless vigour. A correspondent to the BBC on a programme about English grammar in 1983 remarked:

"One reason why the older generation feel so strongly about English grammar is that we were severely punished if we didn't obey the rules! One split infinitive, one whack; two split infinitives, two whacks; and so on."[27]

As a result, the debate took on a degree of passion which the bare facts of the matter never warranted. There was frequent skirmishing between the splitters and anti-splitters until the 1960s. George Bernard Shaw wrote letters to newspapers supporting writers who used the split infinitive, and Raymond Chandler complained to the editor of The Atlantic Monthly about a proofreader who changed Chandler's split infinitives:

"By the way, would you convey my compliments to the purist who reads your proofs and tell him or her that I write in a sort of broken-down patois which is something like the way a Swiss-waiter talks, and that when I split an infinitive, God damn it, I split it so it will remain split, and when I interrupt the velvety smoothness of my more or less literate syntax with a few sudden words of barroom vernacular, this is done with the eyes wide open and the mind relaxed and attentive. The method may not be perfect, but it is all I have."[28]

Post-1960 authorities show a strong tendency to accept the split infinitive. Follett, in Modern American Usage (1966) writes: "The split infinitive has its place in good composition. It should be used when it is expressive and well led up to."[29] Fowler (Gower's revised second edition, 1983) offers the following example of the consequences of refusal to split infinitives: "The greatest difficulty about assessing the economic achievements of the Soviet Union is that its spokesmen try absurdly to exaggerate them; in consequence the visitor may tend badly to underrate them" (italics added). This question results: "Has dread of the split infinitive led the writer to attach the adverbs ['absurdly' and 'badly'] to the wrong verbs, and would he not have done better to boldly split both infinitives, since he cannot put the adverbs after them without spoiling his rhythm" (italics added)?[30] Bernstein (1985) argues that, although infinitives should not always be split, they should be split where doing so improves the sentence: "The natural position for a modifier is before the word it modifies. Thus the natural position for an adverb modifying an infinitive should be just . . . after the to" (italics added). Bernstein continues: "Curme's contention that the split infinitive is often an improvement . . . cannot be disputed."[31] Heffernan and Lincoln, in their modern English composition textbook, agree with the above authors. Some sentences, they write, "are weakened by . . . cumbersome splitting," but in other sentences "an infinitive may be split by a one-word modifier that would be awkward in any other position."[32]

Like most linguistic prescription, disapproval of the split infinitive was originally based on the descriptive observation that it was not in fact a feature of the prestige form of English which those proscribing it wished to champion. This is made explicit in the anonymous 1834 text, the first known statement of the position, and in Alford's objection in 1864, the first truly influential objection to the construction, both cited above. The descriptivist objection involves a person whose idiolect does not have the construction advising against its use on the grounds that it is not the norm: for example, many English speakers avoid split infinitives not because they follow a prescriptive rule, but simply because it was not part of the language that they learned as children. However, as the construction grows in popularity, the strength of the descriptivist objection is progressively reduced.

Many of those who avoid split infinitives differentiate according to type and register. Infinitives split by multi-word phrases ("compound split infinitives") and those split by pronouns are demonstrably less usual than the straightforward example of an infinitive split by an adverb. Likewise, split infinitives are far more common in speech and informal writing than in academic writing. Thus, while an outright rejection of the split infinitive is no longer sustainable on descriptive grounds (as it was in 1834), the advice to avoid it in formal settings, and to avoid some types in particular, remains a tenable position. The prescriptive rule of thumb draws on the descriptive observation that certain split infinitives are not usual in certain situations. For examples of modern linguists using such arguments, see the current views section below.

A second argument is summed up by Alford's statement "It seems to me that we ever regard the to of the infinitive as inseparable from its verb."

The to in the infinitive construction, which is found throughout the Germanic languages, is originally a preposition before the dative of a verbal noun, but in the modern languages it is widely regarded as a particle which serves as a marker of the infinitive. In German, this marker (zu) sometimes precedes the infinitive, but is not regarded as part of it. In English, on the other hand, it is traditional to speak of the "bare infinitive" without to and the "full infinitive" with it, and to conceive of to as part of the full infinitive. (In the sentence "I made my daughter clean her room," clean is a bare infinitive; in "She decided to clean her room," to clean is a full infinitive.) Possibly this is because the absence of an inflected infinitive form made it useful to include the particle in the citation form of the verb, and in some nominal constructions in which other Germanic languages would omit it (e.g. to know her is to love her). The concept of a two-word infinitive can reinforce an intuitive sense that the two words belong together. For instance, the rhetorician John Duncan Quackenbos said, "To have is as much one thing, and as inseparable by modifiers, as the original form habban, or the Latinhabere."[33] The usage writer John Opdycke based a similar argument on the closest French, German, and Latin translations.[34]

The two-part infinitive is disputed, however, and some linguists would say that the infinitive in English is also a single-word verb form, which may or may not be preceded by the particle to. Some modern generative analysts classify to as a "peculiar" auxiliary verb;[35] other analysts, as the infinitival subordinator.[36] Moreover, even when the concept of the full infinitive is accepted, it does not necessarily follow that any two words that belong together grammatically need be adjacent to each other. They usually are, but counter-examples are easily found, such as an adverb splitting a two-word finite verb ("will not do", "has not done").

A frequently discussed argument states that the split-infinitive prohibition is based on Latin. An infinitive in Latin is never used with a marker equivalent to English to, and thus there is no parallel there for the construction. The claim that those who dislike split infinitives are applying rules of Latin grammar to English is asserted in many references that accept the split infinitive. One example is in the American Heritage Book of English Usage: "The only rationale for condemning the construction is based on a false analogy with Latin."[10] In more detail, the usage author Marilyn Moriarty states:

The rule forbidding a split infinitive comes from the time when Latin was the universal language of the world. All scholarly, respectable writing was done in Latin. Scientists and scholars even took Latin names to show that they were learned. In Latin, infinitives appear as a single word. The rule which prohibits splitting an infinite [sic] shows deference to Latin and to the time when the rules which governed Latin grammar were applied to other languages.[37]

The argument implies an adherence to the humanist idea of the greater purity of the classics,[44] which particularly in Renaissance times led people to regard aspects of English that differed from Latin as inferior. However by the 19th century such views were no longer widespread; Moriarty is in error about the age of the prohibition. It has also been stated that an argument from Latin would be fallacious because "there is no precedent in these languages for condemning the split infinitive because in Greek and Latin (and all the other romance languages) the infinitive is a single word that is impossible to sever."[45]

Furthermore, this argument is something of a straw man argument as very few proponents of the rule argue from Latin in any case. Certainly, it is clear that dislike of the split infinitive does not originate from Latin. As shown above, none of the prescriptivists who started the split-infinitive controversy in the 19th century mentioned Latin in connection with it. Occasionally teachers and bloggers can be found who do oppose the split infinitive with such an argument,[46] but it is not found in any statements of the position from the 19th or early 20th century, when the prohibition developed. Of the writers cited here (and the many others consulted) who ascribe the split-infinitive prohibition to Latinism, none cite an authority who condemned the construction on that basis. According to Richard Bailey, the prohibition does not come from a comparison with Latin, and the belief that it does is “part of the folklore of linguistics.”[47]

Present style and usage manuals deem simple split infinitives unobjectionable in many situations.[49] For example, Curme's Grammar of the English Language (1931) says that not only is the split infinitive correct, but it "should be furthered rather than censured, for it makes for clearer expression." The Columbia Guide to Standard American English notes that the split infinitive "eliminates all possibility of ambiguity," in contrast to the "potential for confusion" in an unsplit construction.[50]Merriam-Webster's Dictionary of English Usage says, "the objection to the split infinitive has never had a rational basis."[12] According to Mignon Fogarty, "today almost everyone agrees that it is OK to split infinitives."[51]

Nevertheless, many teachers of English still admonish students against using split infinitives in writing. Because the prohibition has become so widely known, the Columbia Guide recommends that writers "follow the conservative path [of avoiding split infinitives when they are not necessary], especially when you're uncertain of your readers' expectations and sensitivities in this matter."[50] Likewise, the Oxford Dictionaries do not regard the split infinitive as ungrammatical, but on balance consider it likely to produce a weak style and advise against its use for formal correspondence.[52]R. W. Burchfield's revision of Fowler's Modern English Usage goes farther (quoting Burchfield's own 1981 book The Spoken Word): "Avoid splitting infinitives whenever possible, but do not suffer undue remorse if a split infinitive is unavoidable for the completion of a sentence already begun."[53] Still more strongly, the style guide of The Economist says, "Happy the man who has never been told that it is wrong to split an infinitive: the ban is pointless. Unfortunately, to see it broken is so annoying to so many people that you should observe it."[54]

As well as register, tolerance of split infinitives varies according to type. While most authorities accept split infinitives in general, it is not hard to construct an example which any native speaker would reject. Interestingly, Wycliff's Middle English compound split would, if transferred to modern English, be regarded by most people as un-English:

*It was most unkind to in this manner treat their brother.

Attempts to define the boundaries of normality are controversial. In 1996, the usage panel of The American Heritage Book was evenly divided for and against such sentences as,

I expect him to completely and utterly fail

but more than three-quarters of the panel rejected

We are seeking a plan to gradually, systematically, and economically relieve the burden.

Here the problem appears to be the breaking up of the verbal phrase to be seeking a plan to relieve: a segment of the head verbal phrase is so far removed from the remainder that the listener or reader must expend greater effort to understand the sentence. By contrast, 87 percent of the panel deemed acceptable the multi-word adverbial in

We expect our output to more than double in a year

not surprisingly perhaps, because here there is no other place to put the words more than without substantially recasting the sentence.

Although the usage of 'not' in splitting infinitives is an issue that has not attracted much attention from the English users, contemporary English grammar puts the phrase into the same category as above[55] This appears to be because the traditional idiom, placing the negation before the marker (I soon learned not to provoke her) or with verbs of desire, negating the finite verb (I don't want to see you anymore) remains easy and natural, and is still overwhelmingly the more common construction in British English.
On the other hand, in the case of American English, search on the corpus of contemporary American English displays a different aspect.
Its usage is more commonly found in the American context, where we can find at least 2200 cases of the usage of 'not' in splitting infinitives.

Some argue that the two forms have different meanings, while others see a grammatical difference,[13] but most speakers do not make use of such a distinction.

Writers who avoid splitting infinitives either place the splitting element elsewhere in the sentence (as noted in the 1834 proscription) or reformulate the sentence, perhaps rephrasing it without an infinitive and thus avoiding the issue. Considering that many English speakers throughout history have not known the construction, or have known it only passively, there can be no situation in which it is a necessary part of natural speech. However, a sentence with a split infinitive such as "to more than double" must be completely rewritten; it is ungrammatical to put the words "more than" anywhere else in the sentence.[56] While split infinitives can be avoided, a writer must be careful not to produce an awkward or ambiguous sentence. Fowler (1926) stressed that, if a sentence is to be rewritten to remove a split infinitive, this must be done without compromising the language:

It is of no avail merely to fling oneself desperately out of temptation; one must so do it that no traces of the struggle remain; that is, sentences must be thoroughly remodeled instead of having a word lifted from its original place & dumped elsewhere:...[57]

In some cases, moving the adverbial creates an ungrammatical sentence or changes the meaning. R.L. Trask uses this example:[58]

She decided to gradually get rid of the teddy bears she had collected.

"Gradually" splits the infinitive "to get". However, if the adverb were moved, where could it go?

She decided gradually to get rid of the teddy bears she had collected.

This might imply that the decision was gradual.

She decided to get rid of the teddy bears she had collected gradually.

This implies that the collecting process was gradual.

She decided to get gradually rid of the teddy bears she had collected.

This sounds awkward, as it splits the phrase "get rid of".

She decided to get rid gradually of the teddy bears she had collected.

Trask considers this almost as unwieldy as its immediate predecessor.

Gradually, she decided to get rid of the teddy bears she had collected.

This might imply that her decision or the fact that she will get rid of her teddy bears is gradual.

The sentence can be rewritten to maintain its meaning, however, by using a noun or a different grammatical aspect of the verb, or by eschewing the informal "get rid":

The split infinitive, specifically its famous use in the Star Trek opening sequence, is the basis of a joke from Douglas Adams' The Hitchhiker's Guide to the Galaxy: "In those days men were real men, women were real women, small furry creatures from Alpha Centauri were real small furry creatures from Alpha Centauri. And all dared to brave unknown terrors, to do mighty deeds, to boldly split infinitives that no man had split before — and thus was the Empire forged."

Split infinitives are an annoyance to Inspector Fowler (Rowan Atkinson), the main character of the British comedy series The Thin Blue Line. He corrects Constable Goodie for using the famous Star Trek split infinitive only to be chided by his son later in the episode for splitting one himself.

Numerous P. G. Wodehouse characters show the tendency to split infinitives, and other characters, who pride themselves upon being purists, detest this. For example, in The Swoop!, the young hero says triumphantly to his prisoner—a German prince with excellent English—"it is when apparently crushed that the Briton is to more than ever be feared." Even in this dire situation, the prince insists that the protagonist has used bad grammar by splitting an infinitive and has thus spoiled his big speech.[60]

"Are you kidding?" said the communications officer. "I wrote that as fast as I could type it. Probably a minute and a half. You just have to develop an ear for Navy prose, Willie. For instance, note that split infinitive in paragraph three. If you want a letter to sound official, split an infinitive. Use the word 'subject' very often. Repeat phrases as much as possible. See my beautiful reiteration of the phrase 'subject man.' Why, it's got the hypnotic insistence of a bass note in a Bach fugue."

In the television show Psych, S03E05 ("Disco Didn't Die. It Was Murdered!"), in a scene where Shawn and Gus are criticizing the chief's use of idioms, Gus also points out that she splits an infinitive. Ironically, the split infinitive is in a phrase where she encourages the duo to correct her language: "Why don't you tell me how to properly say this."

In "Weird Al" Yankovic's 2014 song "Word Crimes", which is about grammar, the last line ("Try your best to not drool") contains a split infinitive and is thus at odds with Yankovic's penchant for traditional grammar. However, in an interview, he states that he introduced that split infinitive on purpose, "to be ironic, and also to see how many online grammar pedants it would annoy." [61] (The earlier line "...to literally smack a crowbar right upside your stupid head!", which takes issue with a different rule of grammar, also contains a split infinitive.)

^Walsh, Bill (2000). Contemporary Books, ed. Lapsing into a comma: a curmudgeon’s guide to the many things that can go wrong in print—and how to avoid them. Lincolnwood, Illinois: Contemporary Books. pp. 112–113. ISBN0-8092-2535-2.

^Some have suggested that another sentence in Shakespeare, from Coriolanus, Act I, scene 2, contains a split infinitive: "Whatever hath been thought on in this state,/ That could be brought to bodily act, ere Rome/ Had circumvention?" [1], [2] Others say that "bodily" here is an adjective and "act" is a noun, as Vizetelly and Johnson's Dictionary do.

^Jespersen, Otto (1956). Growth and Structure of the English Language. Doubleday. p. 222.

^Richard Lederer, A Man of My Words: Reflections on the English Language, St. Martin's Press, 2003, ISBN 0-312-31785-9, p. 248: "The prohibition of that practice was created in 1762 by one Robert Lowth, an Anglican bishop and self-appointed grammarian." Similarly Peter Stockwell, Sociolinguistics: A Resource Book for Students, Routledge, 2002, ISBN 0-415-23452-2, p. 98.

Hall, Fitzedward (1882). "On the Separation, by a Word or Words, of to and the Infinitive Mood". American Journal of Philology (The Johns Hopkins University Press) 3 (9): 17–24. doi:10.2307/287307. JSTOR287307.